Thank you for your suggestions, carlo_0000, Signaturan, Amstellodamois, Zakairdrop and BitMaker2017.

I already bought the two double molex to 6pin pci-e adaptors. I will try to connect the 6pin cables on the gpu with them and then I will let you know how it went. I also bought one double molex to 8pin pci-e adaptor, in the event that the two 6+2 pin cabels that I already have are to short and I can't use them to power each 8pin of the gpu.

Thank you all once again for your valuable help.

Best regards,
Paulo